Contact materials used in vacuum switches have conventionally been made of, for example, Cu--Cr or Ag--WC. Of these, Cu--Cr for example has excellent circuit breaking performance and withstand voltage performance, but the chopping current is as high as 3 A or more, and the welding separation force is also high. On the other hand, Ag--WC for example has an excellent chopping current of aboiut 1A, but the circuit breaking performance is poor and withstand voltage is low. Cu--Cr contact materials are therefore used mainly in circuit breakers, while Ag--WC contact materials are mainly used in load breakers such as motors.
However, the use different contact materials for different applications as described above necessitates handling of so many types, which is troublesome. In addition, structural modifications have to be made to vacuum switches if the contact material is changed, and likewise to the mechanism and structure of vacuum breakers.
Cu--Cr.sub.2 O.sub.3 is also a known contact material, but as seen from FIG. 4 which is a schematic sectional view of the structure of this material, it has numerous closed pores or voids (7) which render its selectrical performance unstable. In FIG. 4, (6) denotes Cr.sub.2 O.sub.3 and (2) denotes Cu.
If for example this material is used to break large currents, the arc melts the contact surfaces. The surface part of the contact progressively wears down, and a situation in which a void containing residual gas is present close to the contact surface and a situation in which there is not such void close to the contact surface alternately appear. In the first mentioned situation the current breaking fails because the residual gas is blown out when the contact surface melts and the degree of vacuum in the vacuum switch is impaired (the pressure inside the vacuum switch increases). In the second mentioned situation, no gas is blown out upon melting of the contact surface, and the current breaking is therefore successful. Thus, when the device is used to break large currents repeatedly, it fails to perform whenever new voids are destroyed by melting of the overlying surface part of the contact.
If the device is used to break small currents, the arc produced is small and the contact surfaces do not melt as in the case of breaking large currents. However melting does occur in areas where the arc strikes, and if there are voids will residual gas at these points, this gas is released and adversely affects the withstand voltage performance.
The reason why these voids exist is that the wettability of Cr.sub.2 O.sub.3 in Cu is extremely poor, and if the contacts are made by the usual techniques, it is very difficult to reduce the proportion of voids.
The authors of this invention have already carried out experiments with a view to developing contact materials that could satisfy all the above requirements. In for example Japanese Patent Application Kokai Publication No. 1984-215621, a Cu--Cr--Cr.sub.2 O.sub.3 contact material is partially disclosed. Although this contact material gives excellent performance with a view to satisfying all the requirements, it was found in later experiments that its circuit breaking characteristics are not stable and its performance fluctuates.
Conventional vacuum switch contact materials did no therefore have all the requisite chracteristics, and many kinds of materials had to be used for different applications in order that inferior characteristics did not impair contact performance. Further, even if a contact material did have all the requisite characteristics, it lacked stability.